ADM8845 View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

ADM8845

Charge Pump Driver for LCD White LED Backlights

Analog Devices 

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POWER EFFICIENCY

The ADM8845 power efficiency (η) equations are

η = POUT/PIN

PIN = ((VCC × ILOAD × Gain) + (IQ × VCC))

POUT = 6×(VF × ILED)

where:

IQ is the quiescent current of the ADM8845, 2.6 mA.

VF is the LED forward voltage.

Gain is equal to charge pump mode (1×, 1.5×, 2×).

3.4V IIN

VCC

VCC

VOUT

ADM8845

ILOAD

RSET

CTRL1

CTRL2

FB1

FB2

FB3

FB4

ISET GND

FB5

FB6

Figure 34. Charge Pump Power Efficiency Diagram

Examples 1 and 2 show calculations of the ADM8845 power

efficiency; also see Figure 34.

ADM8845

Example 1

The ADM8845 driving six white LED with 20 mA/LED at

VCC = 3.4 V (1.5× mode), LED VF = 4.5 V.

PIN = ((VCC × ILOAD × Gain) + (VCC × IQ))

PIN = ((3.4 × 120 mA × 1.5) + (3.4 × 2.6 mA))

PIN = ((0.612) + (0.00884))

PIN = 0.62084

POUT = 6(VF × ILED)

POUT = 6(4.5V × 20 mA)

POUT = 0.54

η = POUT/PIN

η = 0.54/0.62084

η = 87 %

Example 2

The ADM8845 driving six white LED with 20 mA/LED at

VCC = 3.4 (1.5× mode), LED VF = 3.6 V.

PIN = ((VCC × ILOAD × Gain) + (VCC × IQ))

PIN = ((3.4 × 120 mA × 1.5) + (3.4 × 2.6 mA))

PIN = ((0.612) + (0.00884))

PIN = 0.62084

POUT = 6(VF × ILED)

POUT = 6(3.6 V × 20 mA)

POUT = 0.432

η = POUT/PIN

η = 0.432/0.62084

η = 70 %

Rev. 0 | Page 17 of 20

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